1. Field of the Invention
The invention relates to systems for adjusting spatial plasma densities/distributions and spatial distributions of chemicals within a plasma, and particularly to systems which use a plasma to process a substrate.
2. Discussion of the Background
In many electrical device and solid state manufacturing processes, a plasma reacts, or facilitates a reaction, with a substrate, such as a semiconductor wafer. In order to generate the plasma, power is supplied to a gas by an inductive or a capacitive plasma coupling element. Examples of inductive coupling elements include conductive and helical coils. Many conventional systems supply the RF power through an electrical matching network (MN). One known inductive plasma generating system is disclosed in U.S. pat. No. 5,234,529, issued to Wayne L. Johnson, the inventor of the present application. The contents of that patent are incorporated herein by reference.
One method of generating a plasma source 114 is described with reference to FIG. 1. A gas is supplied to a process chamber 102 through gas inlets 112. An RF power source 110 having an output impedance Rs supplies RF power to a helical coil 104 acting as an inductive coupling element. The coil 104 couples energy into the gas and excites it into a plasma within a plasma region 108 of the process chamber 102. The plasma and energetic and/or reactive particles produced by the plasma (e.g., ions, atom, or molecules), can then be released through an output 120 of the plasma source 114 and used to process a substrate, e.g., a semiconductor wafer 106 or a flat panel display substrate.
During plasma processing, one factor controlling how processing occurs is xe2x80x9cambipolar diffusion.xe2x80x9d The ambipolar diffusion process is illustrated in FIG. 2, which portrays a recombination surface 1306 to which electrons 1302 of the plasma are attracted. Upon reaching the recombination surface 1306, the electrons 1302 adhere thereto, thereby producing a net negative charge which attracts ions 1304 from the plasma. The ions 1304, upon reaching the recombination surface 1306, recombine with electrons 1302 to produce neutral particles 1308. This recombination lowers the ion density np in the plasma since neutral particles 1308 do not contribute to the ion density np. More importantly, the plasma density is reduced adjacent to the recombination surface 1306 as compared to further away from the surface 1306. Consequently, the geometry of the walls acting as recombination surfaces affects the spatial distribution of a plasma within the source. In addition, since some ion species are more susceptible to this recombination process than other species, a recombination surface can cause one or more of the ion species to recombine disproportionately, thereby affecting the chemical composition of the plasma.
The ion density np of the plasma in a particular region is also affected by the rates of several processes, including (1) the rate of production of ion-electron pairs, (2) the rate of recombination of ion-electron pairs, and (3) the rate of flow of electrons and ions into or out of the region (including pumping). The local plasma density np in the region at a particular time is the value at which the aforementioned process rates are at an equilibrium. The value of np also can be affected by the amount of power supplied to the region. More specifically, an increased amount of power supplied to the region tends to increase the local rate of production of ion-electron pairs, thereby increasing the value of np in the region.
Non-uniform spatial distribution of the density of the plasma across the output 120 of the source 114 is disadvantageous. As shown in the graph of FIG. 3A, the local plasma density np at a given location x across the output of a source can depend on the location, as well as the average plasma density  less than np greater than  of the source. The graph includes curves representing np vs. x for two different plasmas, each having its own value of average density  less than np greater than . For both plasmas of this example, np is at a maximum in the center 320 of the source (and, therefore, in the center of the wafer 106) and is smaller at the edges 322. Further, this non-uniformity of np is more pronounced when the average density is higher (high  less than n0 greater than ) than it is when the average density is lower (low  less than np greater than ).
As described above, the ion density np also varies spatially based on the geometry of the source. FIG. 3B is a graph of local ion density np as a function of location x for sources varying effective with and effective length L. As illustrated in the graph, the uniformity of plasma density can depend on the aspect ratio (L/W) of the plasma source. For examply, the ion density np of each of the long and medium sources is greatest in the center 320 of the source and smallest at the edges of 322 whereas, for the short source, np exhibits a relative dip near the center 320. The relative peak in plasma density near the center (and the relatively low plasma density near the edges 322) of a long source, can be caused by the proximity of a side wall 124 to the edge of the source. The side wall provides a recombination surface which increases the rate of recombination of ions and electrons. As a result, the plasma density can be reduced near the edges of a long source.
When processing a substrate, particularly a semiconductor water, non-uniformity of plasma density can cause non-uniformity of reaction characteristics (e.g., reaction rates) across the surface of the substrate. For example, as illustrated in FIG. 3C, if a plasma is used to etch a film on a substrate, and the plasma has a higher density near the center 320 of the wafer 106, the etching rate can be higher in the center of the wafer 106 and lower at the edges 322. Similarly to the example of FIG. 3A, the process of FIG. 3C can exhibit more pronounced non-uniformity in cases of high  less than np greater than  and less pronounced non-uniformity in cases of low  less than np greater than .
The problems of non-uniformity of plasmo densities are discussed in severs U.S. patents which are incorporated herein by reference. Those patents are: U.S. Pat. No. 4,340,461 to Hendricks et al., entitled xe2x80x9cModified RIE Chamber for Uniform Silicon Etchingxe2x80x9d; U.S. Pat. No. 4,971,651 to Wantabe, entitled xe2x80x9cMicrowave Plasma Processing Method and Apparatusxe2x80x9d in which local plasma density is absorbed, attenuated or diffused to produce a uniform plasma density, thereby uniformly processing a wafer; U.S. Pat. No. 5,444,207 to Sekine et al., entitled xe2x80x9cPlasma Generating Device and Surface Processing Device and Method for Processing Wafers in a Uniform Magnetic Fieldxe2x80x9d; U.S. Pat. No. 5,534,108 to Qian et al., entitled xe2x80x9cMethod and Apparatus for Altering Magnetic Coil Current to Produce Etch Uniformity in a Magnetic Field-Enhanced Plasma Reactorxe2x80x9d in which a uniform plasma density is produced by a magnetic field rotating in a plane parallel to a horizontal plane of a processed substrate; U.S. Pat. No. 5,589,737 to Barnes et al., entitled xe2x80x9cPlasma Processor for Large Workpiecesxe2x80x9d in which uneven processing is described as a result of non-uniform plasma density over large workpieces such as rectangular flat panel displays; and U.S. Pat. No. 5,593,539 to Kubota et al., entitled xe2x80x9cPlasma Source for Etchingxe2x80x9d in which electrons are moved in a cycloid motion in order to produce a uniform plasma density.
Improvements in the performance of parallel plasma processors have been made by changing one or more electrodes in various ways. Gorin and Hoog (U.S. Pat. No. 4,209,357) describe increased uniformity of etching using different sized electrodes with adjustable spacing. Adjustable spacing has also been considered by Koch (U.S. Pat. No. 4,340,462). Hendricks et al. (U.S. Pat. No. 4,340,461) describe using a baffle plate to increase the size of the powered electrode. Non-planar electrodes of various shapes have been asserted to be beneficial. Some are simply curved (see Mundt et al. (U.S. Pat. No. 4,297,162) and Mallon (U.S. Pat. No. 5,628,869)); other have more complicated surfaces which may include projections of various shapes (see Zajac (U.S. Pat. Nos. 4,307,283 and 4,342,901) and Salimian et al. (U.S. Pat. No. 5,716,485)). Additionally, known systems achieve a uniform dense plasma using hollow cathodes distributed over the electrode surface while others achieve greater uniformity by selectively spacing wafers from the cathode by using quartz spacers. Electrodes with independently adjustable segments have been proposed. The several segments may be excited by separate RF sources as well. See Susko (U.S. Pat. No. 4,885,074).
Zajac (U.S. Pat. No. 4,307,283), discussed above, also discusses gas flow dynamics in conjunction with electrode shape. A cap with distributed apertures for gas flow and a concave surface facing a wafer to be processed reduces power density at the center of the wafer and, therefore, provides more uniform etching. See Sharp-Geisler (U.S. Pat. No. 4,612,432).
The principal way in which plasma uniformity has been addressed in inductively coupled plasma (ICP) generators is via the excitation coil(s). Varnes et al. U.S. Pat. No. 5,589,737) describes planar coils of relatively complex designs that avoid current and phases non-uniformity associated with coils for which the length exceeds xe2x85x9 wavelength. It is known that coil geometry can cause changes in electron densities. Hook et al. (U.S. Pat. No. 5,648,701) describe coils for use in plasmas at high pressures ( greater than 5 Pa or about 40 mTorr).
An ICP reactor with a plurality of separate concentric channels, each with its own process gas controller and shielded independently powered RF coil, provides improved control of plasma density. See Hartig and Arnold (U.S. Pat. No. 5,683,548). Johnson et al. (U.S. Pat. No. 5,234,529) describe using capacitive shields in ICP reactors to limit the capacitive coupling between the RF coil and the plasma. Moreover, Zarowin and Bollinger (U.S. Pat. No. 5,290,382) disclose an interactive flange which provides a surface separate from the substrate to consume the active species.
Accordingly, there is a need for an apparatus and method which can provide improved adjustment and control of a spatial distribution of a plasma density and/or a spatial distribution of a chemical composition of the plasma. In particular, it is necessary to accurately control the uniformity of a plasma in the vicinity of a substrate, thereby controlling the uniformity, across the surface of the substrate, of a reaction caused by the plasma.
It is therefore an object of the invention to provide a system and method which can adjust the spatial distribution of a plasma and/or the spatial distribution of chemicals within a plasma, particularly a plasma used for processing a substrate.
According to one aspect of the invention, a recombination surface is provided proximate to a selected local region of a plasma, in order to increase a rate of recombination of ions and electrons, thereby reducing a density of at least one chemical component of the plasma, in the selected local region. In particular, by providing a recombination member having a recombination surface of a predetermined geometry and/or material, the distribution of the plasma and/or the distribution of chemicals within the plasma can be controllably adjusted.
According to another aspect of the invention, a conductive shielding element is provided in order to adjust the electric field in the device, thereby controlling a rate of production of ions and free electrons in a selected local region. The conductive shielding element locally reduces an amount of power provided to the selected region of the plasma, thereby reducing the plasma density in the region. The power supplied to the selected region of the plasma is reduced by providing either a conductive element with a current path parallel to an electric field being supplied by a power source, or a conductive loop with a current path encircling a portion of a magnetic field supplied to the plasma. One or more conductive elements can be included in a conductive shielding element, which can be used as an electrical and/or magnetic shield for reducing the amount of power supplied to the plasma in one or more selected local regions.
The invention allows the spatial distribution of the plasma and/or the spatial distribution of chemicals within the plasma to be adjusted, thereby allowing for the control, reduction, or elimination of spatial variations when processing with a plasma. In particular, the spatial variation of the reaction rate or chemistry of a reaction on the surface of a substrate can be controllably adjusted. Consequently, smaller line widths can be achieved, and higher integration densities can be obtained. In addition, rates of device defects can be reduced, resulting in increased manufacturing yield and reduced manufacturing costs.